may

be employed. This turns freely upon its center, and may be pressed by a weight or spring against the teeth. It will be raised as the inclined side of the tooth passes under it by the action of A, and will fall over into the next space, but when A quits the wheel, the detent pressing upon the inclined side of the tooth will move it through a short space backwards, until the point m rests at the bottom of the nook, as shown. The detent thus retains the wheel in its position during the absence of the tooth A. These detents receive other forms, for which I shall refer to the section on Ratchet-work, below.

Fig. 145.

222. A better intermittent motion is produced by a contrivance (fig. 145) which may be termed the Geneva stop, as it is introduced into the mechanism of the Geneva watches.

A is the driver, which revolves continually in the same direction, B the follower, which is to receive from it an intermittent motion, with long intervals of rest. For this purpose its circumference is notched alternately into arcs of circles as ab, concentric to the center of A when placed opposite to it, and into square recesses, as shown in the figure.

A

ს

The circumference of A is a plain circular disc, very nearly of the same radius as the concave tooth which is opposed to it; this disk is provided with a projecting hatchetshaped tooth, flanked by two hollows r and s. When it revolves

(suppose in the direction of the arrow), no motion will be given to B so long as the plain edge is passing the line of centers, but at the same time the concave form of the tooth of B will prevent it from being moved (as in fig. 70).

But when the hatchet-shaped tooth has reached the square recess of B, its point will strike against the side of the recess at d, and carry B through the space of one tooth, so as to bring the next concave arc a b opposite to the plain edge of the disk, which will retain it until another revolution has brought the hatchet into contact with the side of the next recess bf.

The hollow recess at r is necessary to make room for the point d, which during the motion is necessarily thrown nearer to the center of A than the circumference of the plain edge of the latter. The hatchet-tooth being symmetrical will act in either direction. 223. The office of this contrivance in a Geneva watch is to prevent it from being over-wound, whence it is termed a stop; and for this purpose one of the teeth is made convex, as shown in

dotted lines at fg. If A be turned round, the hatchet-tooth will pass four notches in order, but after passing the fourth across the line of centers, the convex edge gf will prevent further rotation, so that in this state the combination becomes a contrivance to prevent an axis from being turned more than a certain number of times in the same direction.

For the wheel A is attached to the axis which is turned by the key in winding, and the wheel B thus prevents this axis from being turned too far, so as to overstrain the spring. As the watch goes during the day the axis of A revolves slowly in the opposite direction, carrying the stop-wheel with it by a similar intermitting motion.

The late Mr. Oldham applied this kind of mechanism to intermittent motions,* and his arrangement is in some respects superior to that of fig. 145. Instead of the hatchet-tooth he employed a pin carried by a plate fixed to the back of the driver, by which means he was enabled to reduce the size of the square notches of the follower.

224. Any required variation in the ratio of angular velocities may be produced by a cam-plate; but if the directional relation is constant the motion will necessarily be limited, as in fig. 122, (page 153). In this contrivance, by altering the form of the curve we may obtain different velocity ratios at every point of its action; as, for example, if a portion of the edge of the cam-plate be concentric to its axis, the pin or bar which it drives will receive no motion while that part of the edge is sliding past it. 225. The curve for a cam of this kind is generally described by Fig. 146.

3

IV

points. The methods of doing this will readily occur in each particular case, but one example may serve to show the nature of the process. In the combination of fig. 122, let the angular velocity ratio vary so that when a series of points 1, 2, 3, 4, 5, fig. 146, in the circumference of the circle C 3, 5 shall have reached in order the point C, the pin in the sliding bar shall be moved into the corresponding positions I, II, III, IV, V. To each of the position points in the circumference of the circle draw tangents, and with center A draw circular arcs in order, each intersecting one of the position points, I, II, III, &c., and the corresponding tangent, as at a, b, c, d, e; thus is obtained a series of points through which, if

d

A

In the machinery of the Banks of England and Ireland.

a curve be drawn, it will be the cam required; for it is manifest, that if any point (as 3) of the circle be brought to C, the corresponding point c of the curve will be moved to III, and thus the pin will be placed in its required position; and so for every other pair of positions.

The curve for a pin of sensible diameter must be obtained from this by the usual method (Art. 130).

226. By means of a properly formed revolving cam-plate a reciprocating motion may be given to a follower which will vary periodically according to any required law.

Thus let A, fig. 147, be the center of motion of a cam-plate nm qp, BD the follower, which in this case is an arm turning on a center B, and furnished with a friction-roller D which rests upon the edge of the cam. But the follower may also be a sliding bar as in fig. 122 (p. 153).

Fig. 147.

B

A

Let A m be the least radius of the cam, and Ap the greatest, and let the radii gradually increase along the edge mn p, and decrease along the edge p q m. Then if the cam revolve continually in the direction of the arrow, the roller D will be by the action of the edge pushed away from the center A, during the passage of m n p under it, and will return to the center during the passage of p q m ; it being supposed to be kept in contact with the edge by weight or by a spring.

In this manner a series of periodic oscillations are communicated to the bar BD, and the velocity ratio of this bar to that of the cam can be adjusted at pleasure to any required law, by shaping the edge of the plate accordingly (Art. 31).

This may be set out by points in the method of which an example has already been given in Art. 225. If the bar be required to remain at rest during a given angular portion of the revolution of the cam, the edge will be an arc of a circle through that angle. If the follower be a straight bar, as in fig. 122, and

this bar be required to perform its motion in both directions with a constant angular velocity ratio to that of the cam, then must a cam-plate be formed of two of the curves given in Art. 203, each occupying half the circumference, and set back to back, so as to produce a heart-shaped figure.

227. If the cam-plate be required to communicate more than one double oscillation in each revolution, its edge must be formed into a corresponding number of waves, as A, fig. 148; and if the follower is to be raised gently and let fall by its own weight, the waves must terminate abruptly, as in B. If the follower is to

receive a series of lifts with intervals of rest, the cam becomes a set of teeth projecting from the circumference of a wheel, as in D. When the cam is employed to lift a vertical bar or stamper, these separate teeth are often termed wipers or tappets.

228. The axis of the follower, if it be a revolving bar, as in fig. 147, is not necessarily parallel to that of the cam; but may be set at any angle to it, if the bar revolve only through a small angle, whose tangent in the mean position is in the plane of rotation of the cam.

229. The simplest form of a cam is that of an excentric circle, as at C, fig. 148. Let a be the excentric center of motion, b the center of the cam, ac the direction of motion of the follower, which is a roller whose center is c. Then be is plainly constant, and the motion given to the follower the same as if a link be and crank ab were employed.

230. If the weight or spring be inconvenient, the cam may be made to press the follower in both directions by means of a double curve. This cannot be made in the form of a slit, as in fig. 122, because the motion is now to take place indefinitely in the same direction; but a groove in the face of a plate may be employed, as at A, fig. 149.

231. If the cam revolve always in the same direction, the outside curve is only required during that portion of the motion in which the follower approaches the cam, and it may be supplied